[Elecraft] Tips for using APF

[Bill W4ZV]

Very nice summary Wayne!  I'd add a couple of comments:

1.  I find an even wider WIDTH works better for me (400-450 Hz using a 500
Hz 8-pole).  If I set WIDTH too narrow (e.g. 50-100 Hz) I get too much
ambient ringing.
2.  Tuning is indeed very critical.  This is why I only use APF for
*extremely* weak signals buried in noise.  I never leave it on continuously
and it would be unusable in a fast-paced activity like a contest.
3.  I reduce my AF Gain from its normal setting.  APF compensates with Gain
of ~9 dB at its exact center so the signal will "pop up" when you get the
critical tuning correct.
4.  Indeed the effectiveness of APF seems to vary with the type of noise.  I
find it most useful in mild atmospheric noise (lightning from a long
distance) and less useful in galactic or white noise.

73,  Bill  W4ZV


wayne burdick wrote:
> 
> A couple of notes from the Elecraft lab:
> 
> 1. When using APF, it's critical to tune the signal in very  
> accurately. (Otherwise it may seem like APF isn't working.) For best  
> results, I recommend using 1-Hz VFO tuning. An alternative is to  
> adjust the APF center pitch, which can be moved in 5-Hz increments  
> using the SHIFT control. Note that only 10-Hz increments are displayed.
> 
> 2. There are two filter "Presets" (I and II, accessed by HOLDing the  
> HI/WIDTH knob). You can turn APF on for one preset and off for the  
> other. This control is closer to where the action is than DUAL PB, and  
> I find it more convenient to use.
> 
> 3. You can of course hear the effect of APF by ear, but if you want to  
> quantify it, try using the built-in AF voltmeter on a very weak  
> constant carrier buried in noise. Here's the procedure I use:
> 
>    a. Switch to a noisy band/antenna (80 m at night, let's say, with a  
> vertical or inverted-V).
>    b. Turn APF off.
>    c. Set WIDTH to 300 Hz, with no SHIFT.
>    d. Hunt for an extremely weak on-air carrier -- one you can just  
> barely detect. A carrier that's caused by a local computer or TV is  
> ideal because there won't be much fading during the test.
>    e. Tap DISP and rotate VFO B until you get to AFV (audio volt  
> meter). Let the reading stabilize.
>    f. Rotate VFO B one more tic clockwise to dBV. The number will  
> bounce around 0.0 by maybe +/- 1 dB. Call this the reference level.
>    g. Now tune well off the carrier so that you hear only noise. The  
> dBV number will indicate how many dB the signal has dropped. If it  
> drops 3 dB, your S+N/N ratio is 3 dB.
>    h. Tune the signal back in and make sure the level is back to  
> around 0.0.
>    i. Turn on APF. Using FINE VFO tuning (1-Hz steps), make sure the  
> carrier is peaked as indicated by ear or on the dBV indication.
>    j. Use VFO B to go back to AFV for a few seconds to capture a new  
> relative signal level. Then go back to dBV, which should again show  
> about 0.0 (bouncing a little).
>    k. Tune well off the carrier so you again hear only noise.
>    l. Note the new (negative) dB reading.
> 
> I just repeated this little test on 80 m with persistent S-6 noise.  
> Here's what I got:
> 
>    Without APF:  1 dB S+N/N
>    With APF:     6 dB S+N/N
> 
> In fact, having turned APF on, I was now able to find a lot more  
> little carriers :)  The secret is to tune slowly. Try hunting for DX  
> with APF on at the low end of 40 or 80 m when the sun is just setting.
> 
> 73,
> Wayne
> N6KR
> 
> 
> 
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